linux-kernel - Re: [RFC] perf probe: Fix offline module name missmatch issue

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Date:	Tue, 26 Apr 2016 17:15:43 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>,
	Masami Hiramatsu <mhiramat@...nel.org>
CC:	<linux-kernel@...r.kernel.org>, <acme@...nel.org>,
	<peterz@...radead.org>, <mingo@...hat.com>,
	<alexander.shishkin@...ux.intel.com>,
	<masami.hiramatsu.pt@...achi.com>, <namhyung@...nel.org>,
	<srikar@...ux.vnet.ibm.com>, <naveen.n.rao@...ux.vnet.ibm.com>
Subject: Re: [RFC] perf probe: Fix offline module name missmatch issue



On 2016/4/26 16:56, Ravi Bangoria wrote:
> Thanks Masami for reviewing.
>
> Please find my replies to your comment.
>
> On Tuesday 26 April 2016 02:54 AM, Masami Hiramatsu wrote:
>> Hi Ravi,
>>
>> On Mon, 25 Apr 2016 16:08:27 +0530
>> Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com> wrote:
>>
>>> Perf can add a probe on kernel module which has not been loaded yet.
>>> Current implementation finds module name from path. But if filename
>>> is different from actual module name then perf fails to register
>>> probe while loading module because of mismatch in names. For example,
>>> samples/kobject/kobject-example.ko is loaded as kobject_example.
>> Ah! right, good catch!
>> Have some comment below;
>>
>>> diff --git a/tools/perf/util/probe-event.c 
>>> b/tools/perf/util/probe-event.c
>>> index 8319fbb..05d0905 100644
>>> --- a/tools/perf/util/probe-event.c
>>> +++ b/tools/perf/util/probe-event.c
>>> @@ -265,6 +265,65 @@ static bool kprobe_warn_out_range(const char 
>>> *symbol, unsigned long address)
>>>       return true;
>>>   }
>>>   +/*
>>> + * NOTE:
>>> + * '.gnu.linkonce.this_module' section of kernel module elf directly
>>> + * maps to 'struct module' from linux/module.h. This section contains
>>> + * actual module name which will be used by kernel after loading it.
>>> + * But, we cannot use 'struct module' here since linux/module.h is not
>>> + * exposed to user-space. Offset of 'name' has remained same from long
>>> + * time, so hardcoding it here.
>>> + */
>>> +#ifdef __LP64__
>>> +#define MOD_NAME_OFFSET 24
>>> +#else
>>> +#define MOD_NAME_OFFSET 12
>>> +#endif
>>> +
>>> +/*
>>> + * @module can be module name of module file path. In case of path,
>>> + * inspect elf and find out what is actual module name.
>>> + * Caller has to free mod_name after using it.
>>> + */
>>> +char *find_module_name(const char *module)
>> Could you make this function static, since there is no caller outside
>> this file?
>
> Yes. no caller outside of this file. But,
>
> In this patch, function find_module_name is defined outside of
> #ifdef HAVE_DWARF_SUPPORT while it's being called from inside of
> #ifdef HAVE_DWARF_SUPPORT.
>
> If I make it static and if there is no dwarf support, there will be 
> compilation
> error about function defined but not used.
>
> And in second patch("perf probe: Fix module probe issue if no dwarf 
> support"),
> I'm calling it from outside of #ifdef HAVE_DWARF_SUPPORT.
>
> So I have two options:
> 1. merge both the patches and make definition as static
> 2. make function static in second patch
>
> I've chose second approach and sent v2. But please let me know if 
> there is
> better way to do it.
>

Try __maybe_unused directive?

Thank you.